1. Field of the Invention
The present invention relates to a semiconductor device which is formed by using a semiconductor film having a crystal structure (also referred to as a crystalline semiconductor film) and a method for manufacturing the same, as well as a semiconductor integrated circuit equipped with a circuit integrating the semiconductor devices and a method for manufacturing the same. Further, the present invention relates to a semiconductor device having a plurality of the semiconductor integrated circuits. And, the present invention especially relates to a thin film transistor which forms a channel forming region by a crystalline semiconductor film formed on an insulating surface as a semiconductor device.
2. Description of the Related Art
A technique for forming a semiconductor device such as a thin film transistor and the like using a crystalline semiconductor film formed on an insulation substrate such as glass, has been developed. The thin film transistor formed by using the crystalline semiconductor film is applied to a semiconductor integrated circuit, and the semiconductor integrated circuit is utilized to a flat panel display typified by a liquid crystal display device and an EL (electro luminescence) display device.
A current mirror circuit is a basic circuit of a semiconductor integrated circuit having the thin film transistor. The presupposition of the current mirror circuit is to have two thin film transistors with same electric characteristics. Such a circuit structure also can be given as an operational amplifier, a differential amplifier and the like.
As a method for forming a crystalline semiconductor film on an insulation substrate, a technique by using laser beams to crystallize an amorphous semiconductor film has been developed. In a semiconductor manufacturing process, such as the technique for crystallizing an amorphous semiconductor film by using laser beams, as light sources of the laser beams, a gas laser such as an excimer laser, and a solid laser such as a YAG laser are used generally. An example for crystallizing an amorphous semiconductor film by irradiating laser beams is disclosed in JP-A-62-104117, which proposes a poly-crystallization of the amorphous semiconductor film by high-speed scan with the scanning speed of laser beams set to more than a diameter of beam spot×5000/second, without making the amorphous semiconductor film result in a perfect melting state. In addition, U.S. Pat. No. 4,330,363 discloses a technique to form a substantially single crystal region by irradiating the extended laser beams on a semiconductor film, which is formed in the shape of an island. Or a method of irradiating a laser beam formed into a linear-like shape by an optical system, such as a laser processing apparatus disclosed in JP-A-8-195357 is known.
Further, JP-A-2001-144027 discloses a technique such that crystalline semiconductor films with large grain size are formed by irradiating laser beams of a second harmonic onto the amorphous semiconductor films using solid laser oscillation apparatus such as Nd: YVO4 laser. A transistor is thus constituted.
However, when crystallization is made by irradiating the laser beams onto the amorphous semiconductor film, the crystal includes poly-crystals, producing defects such as a grain boundary which is formed at random. Therefore, it becomes difficult to obtain uniform crystallinity and crystal orientations. As a result, current values may vary even when semiconductor devices in same size are made and same voltages are applied to the semiconductor devices.
Crystal defects are involved in a grain boundary, resulting in a carrier trap. This may be considered as a causative factor that mobility of electrons or holes fall. Also, it is impossible to form a semiconductor film with neither distortion nor crystal defects due to a volume shrinkage of the semiconductor films, a thermal stress applied between the semiconductor film and a base film, or lattice mismatching, accompanied with crystallization. Consequently, the distortion and crystal defects produce causative factors of not only variation of electrical characteristics of the semiconductor device, but also inferior electrical characteristics of the semiconductor device.
Especially when crystalline semiconductor films are formed by using laser beams on a non-alkali glass substrate used abundantly and industrially, the focuses of the laser beams varies in response to the influence of the surge of the non-alkali glass substrate itself, involving a problem of causing crystalline variation as a result.
Furthermore, in order for a non-alkali glass substrate to avoid contamination by the alkaline metal, it is necessary to prepare protection films such as insulating films, as base films. And it is almost impossible to form thereon the crystalline semiconductor films with no grain boundary and crystal defects, which are eliminated.
The semiconductor integrated circuit and the like have semiconductor films formed on cheap glass substrates to constitute transistors, therefore, it is almost impossible to arrange transistors so as to aovid grain boundaries formed at random. That is, the grain boundaris or crystal defects involved unexpectedly could not be eliminated by controlling strictly the crystallinity of the channel forming regions of transistors. This produced a causative factor of variation of electrical characteristics of the semiconductor device, thus, it is difficult to form a circuit which requires high conformity (for example, a current mirror circuit).